In the case of load-bearing joints subjected to static loading, for example in the chassis area of a motor vehicle, in certain situations (such as periods of rest in combination with only minimal joint movements, or freeway driving) the tribological system of the joint is particularly severely loaded, and in some circumstances this can result in premature wear or failure of the joint. This is related to the fact that in these situations, lubricant present in the ball joint is partially expressed out of the contact area between the ball pin and the bearing shell due to the high load in combination with at most small movements of the joint. Because of this the mixed friction that always exists in a ball joint, namely between fluid friction and solid-on-solid friction, is displaced more markedly toward solid-on-solid friction, with corresponding disadvantageous consequences for the wear of the joint. Another problem known to affect ball joints is corrosion, particularly in the area of the ball pin sealed by sealing bellows, and—if moisture penetrates into the ball joint—also in the bearing gap between the joint ball and the ball socket. This too increases the wear of the joint and can result in the onset of joint play and ultimately in failure of the joint.
From the prior art corrosion-protected ball pins and also ball pins with surfaces protected against wear or ball pins with greater fatigue strength are known. According to the prior art enhanced fatigue strength and wear resistance are primarily achieved by improving the material properties by heat treatment or by hardening of the surface layer, in the latter case particularly on the surface of the ball pin.
A known measure against corrosion in the area of the ball pin that is sealed by the sealing bellows or in the contact zone between the joint ball and the ball socket consists in the use of nitrided and then oxidized ball pins. According to existing knowledge nitriding of the ball pin serves directly to enhance corrosion protection, while the subsequent oxidation seals the pores produced during nitriding, which increases the corrosion resistance still more. Thus, according to existing knowledge the oxidation subsequent to nitriding serves only to improve the corrosion properties of an otherwise bare steel ball pin.
In the treatments known from the prior art nitriding and surface layer hardening are mutually exclusive, since either the surface hardening is impaired due to the high process temperatures required for nitriding, or conversely, prior nitriding and hence the corrosion protection are destroyed again by a surface layer hardening process. The problem is that in relation to high fatigue strength or load-bearing and wear resistance as well, the desired surface layer hardening of the ball pin should typically take place in the same surface areas of the pin which should also be protected by the nitriding treatment, namely primarily in the area of the joint ball and if necessary in the area of the ball pin that is sealed by the sealing bellows.
In summary therefore, the problem is that on the one hand the nitriding of the ball pin desired with a view to corrosion protection, and on the other hand the surface layer hardening desired with a view to long life and good wear resistance, have until now been mutually exclusive for a ball pin—at any rate so far as was known from the prior art.